The invention relates to formation of objects, having net-shaped and other complex geometries, from aluminum and its alloys with particular reference to powder metallurgy and metal injection molding.
Aluminum and its alloys are commonly used in many applications such as cooking utensils, industrial components, photographic reflectors and storage equipment. These materials have several very important desirable attributes such as light weight, high thermal conductivity, non-magnetic, high strength-to-weight ratio, which are not commonly found in other metal alloys.
For cooking utensils, its light weight, high thermal conductivity and high corrosion resistance make it very attractive for food preparation. For industrial components, its excellent corrosion resistance, high thermal conductivity and superior strength-to-weight ratio allow many important applications such as actuator arms in hard disk drive, heat sink and electronic casings. For photographic reflectors, it offers the advantages of high light reflectivity and non-tarnishing characteristics. Furthermore, the non-magnetic characteristics makes aluminum useful for electrical shielding purposes such as bus-bar housings or enclosures for other electrical equipment.
These aluminum and aluminum alloys in various applications can be processed in many different ways. For example, a shape and investment casting process can offer design flexibility with low capital investment but the method is not suitable for large volume production because a new mold is required for each cast piece. Die casting offers high volume capability and design flexibility but the finished part is prone to internal porosity, blow holes and undesirable flashing. Extrusion processes are simple but the geometry is very limited. In forging, the process offers good mechanical properties but limited shape complexity and additional secondary operations needed. Thus, all these processes are limited when applied to the production of miniaturized components in large volumes.
Another metal forming process is powder metallurgy where a metal powder is used and shaped into finished parts that meet the dimensional specifications of the finished article along with excellent shape complexity, minimal level of porosity and little or no material wastage. Powder metallurgy is well known in this field but shape complexity is restricted by the die compaction geometry and the powder flowability.
Metal Injection Molding (MIM) is another known field with many patents filed and issued over the last 20 years. However, these tend to be limited to common, less reactive, materials such as iron, stainless steels, low-alloy steels and tungsten alloys. When used in a metal injection molding process, aluminum in powder form is found to be reactive, rapidly forming surface oxide films. As a result good mechanical properties and low-impurity bodies are difficult to obtain, regardless of what sintering process is employed. These oxide films are not easily removed or reduced. For this reason, processes for producing net-shaped and complex parts via aluminum powder are limited. While powder metallurgy pressing operation may provide high green strength through sufficient pressure, metal injection molding is not known to produce metal parts from aluminum powder.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 4,623,388 describes a process for producing a composite material. A matrix of aluminum reinforced by silicon carbide particles. The concentration of silicon carbide was much greater than concentrations used to promote sinterability (as in our invention). Other examples of aluminum-alloy composite can be found in U.S. Pat. No. 4,973,522 and in U.S. Pat. No. 6,077,327. In these processes the purpose of adding silicon carbide into aluminum is for high pressure compaction (mold temperature has to be higher than melting point of aluminum, 660xc2x0 C.). This is not applicable to the present invention where mold temp is not more than 150xc2x0 C. These processes seek to enhance thermal conductivities in the sintered composite. They represent a powder metallurgy process where the green part already has very high density (about 90-95%) but shape geometry is very limited. They require the addition of silicon carbide has to be substantial to see the effect.
In U.S. Pat. No. 5,057,903, the use of aluminum and silicon carbide particles is to promote thermal conductivities in thermoplastic based material, while U.S. Pat. No. 6,346,133 describes metal based powder compositions containing silicon carbide as an alloying powder. Here silicon carbide is added into iron-based or nickel based powder, under high pressure and high temperature compaction, to enhance strength, ductility, and machine-ability.
In U.S. Pat. No. 3,971,657, Daver teaches production of sintered bodies of particulate metal, especially porous sintered bodies, from particles of metal having a refractory oxide coating. A minor proportion of a flux is mixed with the particulate metal before sintering to aid in removing oxide from surfaces of the metal particles. The particulate metal may be aluminum, with which there may be mixed a minor proportion of particles of an alloying element. The flux may be a mixture of potassium fluoaluminate complexes; the residue of this flux, after sintering, provides a coating that aids in protecting the sintered article against corrosion. An important feature of the Daver process is that the product after sintering has high porosity (and low density). In fact, one application of the process is for the production of filters.
In U.S. Pat. No. 6,262,150 entitled xe2x80x9cFeedstock and Process for Metal Injection Moldingxe2x80x9d, it is reported that new binder additives can enhance solid loading for many materials including aluminum, but aluminum in powder form, as mentioned earlier, is reactive and will not exhibit good sintering behavior, particularly since exposure to water is required to remove the binder.
It has been an object of at least one embodiment of the present invention to provide a process for manufacturing aluminum, and aluminum alloy, objects of small size and intricate shapes.
Another object of at least one embodiment of the present invention has been that said process be based on metal injection molding.
Still another object of at least one embodiment of the present invention has been that said process be compatible with metal injection molding as practiced for other materials.
These objects have been achieved by mixing a composition of elemental powders into a feedstock that includes aluminum in the amount of at least 95% by weight, the rest being silicon carbide or a metallic fluoride in an amount sufficient for the required density and strength. The process includes molding the feedstock into the form of compacted items such as heat sink and then sintering the compact items at sintering temperature of between 600xc2x0 C. and 650xc2x0 C.
The sintering temperature of the alloy is between 600xc2x0 C. to 650xc2x0 C. in either vacuum or nitrogen or argon atmosphere. In the desired alloy, it comprises approximately 97% by weight of Al, and the rest 3% by weight of silicon carbide or metallic fluorides with a sintering temperature of between 600xc2x0 C. and 650xc2x0 C. and a sintering time of approximately 60 minutes in a vacuum atmosphere of  less than 0.01 torr.
The technical advantage of the aluminum alloy of the present invention is that it is relatively easy to source for the alloys. Aluminum, Silicon Carbide and metallic fluorides are easy to buy from powder manufacturers worldwide.
The aluminum alloys of the present invention can be easily manufactured in large volume economically in many intricate shapes and sizes.
Another technical advantage of the present invention is that it can be net-shaped with excellent dimensional control and mechanical properties. Little or no secondary operation is necessary to the finished parts. Further, the present invention allows the manufacture of miniaturized complex geometry of less than 1 g, wall thickness of less than 0.3 mm and surface finish of less than 0.5 microns.